Welcome to BYC.
The Usual Guidelines
For each adult, standard-sized hen you need:
- 4 square feet in the coop (.37 square meters)
- 10 square feet in the run (.93 square meters),
- 1 linear foot of roost (.3 meters),
- 1/4 of a nest box,
- And 1 square foot (.09 square meters) of permanent, 24/7/365 ventilation, preferably located over the birds' heads when they're sitting on the roost.
12 hens
- 48 square feet in the coop. 6'x8' is more practical than 4'x12' since a long, skinny coop like that would be difficult to work inside.
- 12 feet of roost
- 120 square feet in the run. 10'x12' or 8'x15' -- 8'x16' means fewer odd cuts than either of those. 6'x20' is possible, especially if your run is an open-topped, fenced area instead of fully-enclosed with a solid and/or wire roof but risks social problems because subordinate hens need to be able to pass the dominant hens at a respectful distance.
- 12 square feet of ventilation.
- 3 nest boxes.
That said, those are *guidelines*, not hard-and-fast *rules*.
BUT, you will need additional space for integration of new birds into the flock. So if you can go bigger, that's better.
Why 10x5 when lumber comes in multiples of 4 feet? You could build 12x6 with the same lumber, using 12-foot boards and cutting them in half for the short side instead of throwing away several feet of each board after cutting?
Some other thoughts,
Florida is an excellent place for an Open Air coop -- essentially a roofed run with a 3-sided shelter on the windward side. These are less expensive to build than traditional enclosed coops.
Here's my article on hot climate chicken-keeping:
https://www.backyardchickens.com/articles/hot-climate-chicken-housing-and-care.77263/
My article on ventilation:
https://www.backyardchickens.com/articles/repecka-illustrates-coop-ventilation.77659/
Here are some useful articles on integration:
https://www.backyardchickens.com/articles/integrating-new-birds-at-4-weeks-old.72603/
https://www.backyardchickens.com/articles/my-coop-brooder-and-integration.74591/
https://www.backyardchickens.com/articles/introducing-new-chickens-using-the-“see-but-don’t-touch”-method.67839/
https://www.backyardchickens.com/articles/adding-to-your-flock.47756/
And an excellent article on why those numbers above are *guidelines*:
https://www.backyardchickens.com/articles/how-much-room-do-chickens-need.66180/